A para-aramid is a polymeric material with excellent crystallinity and excellent resistance to heat. Preparations of para-aramid and PPTA films are disclosed PPTA films in U.S. Pat. No. 4,857,255 and U.S. Pat. No. 4,752,643, and para-aramid films composed of a para-aramid consisting of aromatic rings having a nuclear substituting radical are disclosed in Japanese Unexamined Patent Publications No. 58-42649, No. 59-45124, and No. 62-70421.
In the meantime, there are proposed preparations of para-aramid fibers in U.S. Pat. No. 3,819,587, No. 3,767,756, No. 3,869,429, No. 4,016,236, No. 4,419,317, and No. 4,374,978. These known para-aramid shaped articles have generally excellent mechanical properties and heat resistance properties. Accordingly, applications of the shaped articles have been attempted in various fields of use. It has been a problem, however, that para-aramid shaped articles exhibit a great moisture absorption and a high hygroscopic expansion coefficient. A para-aramid shaped article tends to suffer a dimensional change due to moisture absorption (hygroscopic absorption). Problems are encountered in various applications of the shaped articles; for example, a composite material made of PPTA fiber will exhibit crack formation, which results from a repetition of moisture absorption-desorption cycles, and applications of the film as a high-density magnetic recording medium, high-density printed circuit, or the like, will exhibit problems, because the film is required to exhibit dimensional stability.
There have been proposed attempts to obtain PPTA films having improved hygroscopic properties by promoting orientation and crystallization of PPTA by means of extensive heat treatment and/or stretching in Japanese Unexamined Patent Publications No. 62-246719 and No. 6-136156. In Japanese Unexamined Patent Publication No. 62-246719, for example, a process for preparing a film is described where a raw film obtained through coagulation and washing is treated by heating under tension at 300.degree.-500.degree. C. Although application of heat-treatment at high temperature in the known processes can produce films having a low coefficient of hygroscopic expansion, the heat treated films have excessively reduced elongation and tend to become brittle. Satisfactory improvement of films in hygroscopic expansion coefficient cannot be obtained by these known processes when elongation of the films must be maintained at a desired level.
In order to prevent deterioration of the mechanical properties of film during heat-treatment, an acid component contained in a shaped article after coagulation should be removed as completely as possible, not only in the case where a film is cast directly from a polymerization reaction mixture obtained by polymerizing a diacid chloride with a diamine, but also in the case where a film is cast from a dope prepared by dissolving a polymer separated from the reaction mixture in a strong inorganic acid, such as concentrated sulfuric acid. It is noted that satisfactory removal of an acid component in a shaped article cannot be successfully accomplished only by conventional washing with water or warm water. In Japanese Unexamined Patent Publication No. 50-102650, for an example, there is described a preparation of a film containing 500 ppm or less of an ionic inorganic compound by washing. However, deterioration of the mechanical properties of the film tends to occur when heat is applied to the film during film making or in the event of application, since a minute amount of acid, either derived from the acid generated during polymerization or introduced into a dope for casting as a solvent, is difficult to remove even by this process.
In Japanese Unexamined Patent Publication No. 62-134229, a method of washing a coagulated film with water for effective removal of an acid component is described in which the coagulated film is washed after it has been neutralized by aqueous alkaline solution. In a process for producing fiber, a similar method is proposed in Japanese Unexamined Patent Publication No. 58-91809, in which PPTA fibers as spun from a concentrated sulfuric acid dope are piled up on a net conveyor while the fibers are neutralized before they are washed with a low-ion concentration water.
As explained above, in a process for making a para-aramid shaped article having an improved hygroscopic property by enhancing the crystallinity of the para-aramid by heat treatment, generally, a strong acid component, such as sulfuric acid or hydrochloric acid, generated during polymerization of the para-aramid or introduced into a dope for casting, is completely removed by neutralizing with an aqueous alkaline solution, and then the salt formed by the neutralization is washed off.
In the meantime, para-aramid prepared by conventional polymerization contains a carboxyl end group which is derived from the monomeric aromatic dicarboxylic acid, and further it sometimes contains a sulfonic group as a substituent of a benzene ring in the case where a shaped article is cast from a concentrated sulfuric acid solution. These acid groups (alkali reactive groups) turn into salt forms with alkali metals combined at their ends when they are neutralized in an aqueous alkaline solution. Since the salts are stable in the vicinity of neutrality, the metal ion combined with the end groups cannot be removed by washing in water after a neutralizing step. Accordingly, a para-aramid shaped article, prepared by casting which involves conventional neutralization by an alkali, contains a metal ion combined with the acid end group. A content of the acid end group varies with a feed ratio of monomer in the polymerization; in the case of a carboxyl group, the content is normally 40-80 meq/Kg, and in the case of a sulfonic group, the content is 0-10 meq/Kg. Therefore, in the case where the acid end groups are combined with an alkali metal, for example sodium, metal ions (sodium ions) by weight are contained in an amount of from about 1000 ppm to about 2000 ppm.
In the prior art technique, no attention has been paid to the influence of a relatively minute amount of metal ions combined with the acid end groups on an improvement in hygroscopic properties of a para-aramid shaped article. This is because the influence of the presence of a relatively minute amount of combined metal ion on the hygroscopic properties of a para-aramid shaped article has not been appreciated and because no efficacious method of extracting the combined metal ion contained in para-aramid has been available in the art.
Regarding removal of the contaminant, Japanese Unexamined Patent Publication No. 62-115036 proposes a process for preparing PPTA film in which the film is, after being neutralized, washed or rinsed in warm water in the presence of ultrasonic irradiation (ultrasonic cleaning).
This known washing method can remove metal ions and counter-ions of inorganic salts and alkaline compounds included in a shaped article, but it cannot remove the metal ions combined by ionic bonds with acid groups, such as the carboxyl end group or the like. Accordingly, the method cannot improve the hygroscopic properties of para-aramid shaped articles. A similar result is obtained in the case where a polymerization reaction mixture is supplied after being neutralized for film casting.
As explained above, a para-aramid shaped article having a sufficiently improved hygroscopic expansion coefficient, while maintaining excellent mechanical characteristics of the shaped article, is yet unknown.
As explained above, a para-aramid shaped article according to the prior art, though being excellent in mechanical characteristics and heat durability, exhibits a large hygroscopic absorption (moisture absorption) as well as a high hygroscopic expansion coefficient. A para-aramid shaped article is liable to give rise to dimensional change by moisture absorption, and in its use, this dimensional change invites problems in applications where dimensional stability of the shaped article is required.
Accordingly, the present invention provides a para-aramid shaped article in which moisture absorption and the hygroscopic expansion coefficients are improved without losing its excellent mechanical properties, and a process for preparing the same.